The present invention relates to an information communication technique using light signals and to an optical communication apparatus and an optical communication system suitable for speeding-up of signal transmission and an increase in the capacity for the transmission of a signal or an increase in reliability of signal transmission.
The development of research toward the speeding-up of signal transmission and an increase in the capacity for the signal transmission has recently been active in an information communication technique (hereinafter called optical transmission technique) which performs the transmission of a light signal through an optical fiber. An optical communication system wherein the transmission rate reaches about 2.4 Gb/s (giga-bit/s) at present, has been put into practical use upon the transmission of a light signal between base stations (communication equipment centers), for example. Further, the development of research intended for a high-speed capacity increase of 40 Gb/s or more is in progress.
On the other hand, the introduction of a so-called optical communication system for replacing a transmission signal with light is proceeding even at information communications made between base stations and users (home, office, etc.), and an increase in the reliability of an optical network unit (ONU) on the user side remains a problem. This problem is of importance even to an optical communication apparatus adopted in the transmission of a light signal between the base stations.
The speeding up of the light signal and the great increase in the capacity for the transmission thereof in the optical communication system are problems which are integrally inseparably related to each other. The above-described transmission rate is known as one reference for evaluating these performance. Parameters for improving the transmission rate exist in specifications of, for example, a transmitting device or transmitter for transmitting a light signal, a receiving device or receiver for receiving the light signal therein, and a signal processing device for converting the received signal to an electric signal or decoding it. As to the transmitting device, the shortening of each pulse of a light signal generated according to an electric signal (including an electric signal temporarily converted from a signal inputted by light) without impairing the S/N ratio (signal-to-noise ratio) is taken as one parameter (the present parameter will hereinafter be called xe2x80x9cfirst viewpointxe2x80x9d). Further, one of other parameters is that a plurality of light signals different in wavelength from each other are used and signals to be transmitted are distributed and transmitted every wavelength (hereinafter the present parameter will hereinafter be called xe2x80x9csecond viewpointxe2x80x9d). Thus, the background art related to the improvement in the transmission rate in the optical communication system will be viewed from the first and second viewpoints respectively and summarized below.
1. First Viewpoint (Shortening of Pulse of Light Signal)
In view of trends of the background art from the first viewpoint, an improvement in performance of a so-called optical modulator for modulating light oscillated from a laser light source according to a transmission signal is now mainstream. The present optical modulator is constructed as a so-called semiconductor optical device formed by stacking semiconductor layers on each other. Most of the optical modulators are (monolithically) formed over the same substrate as a semiconductor laser device used as the laser light source. Most of these optical modulators intermittently apply an electric field to a semiconductor layer (hereinafter called xe2x80x9cwave-guiding layerxe2x80x9d) for wave-guiding laser light or a semiconductor layer (hereinafter called xe2x80x9ccladding layerxe2x80x9d) jointed thereto, modulate an absorption coefficient of at least one of the semiconductor layers with respect to the laser light, and repeat transmission and cutting off of a light signal incident to an optical fiber corresponding to a signal transmission line.
The principle of operation of this type of optical modulator is that a forbidden band width Eg of each semiconductor layer is reduced by the application of the electric field, and the semiconductor layer is allowed to absorb light having a much longer wavelength as compared with light xcexg of the longest wavelength (corresponding to the forbidden band width of the corresponding semiconductor layer) originally absorbable by the semiconductor layer. This principle will be explained typically with reference to FIG. 23. The following relation is established between a forbidden band width Eg (unit: eV) of each semiconductor layer and the longest wavelength xcexg (unit: xcexcm) of light absorbed thereby:
Eg=1.24/xcex1gxe2x80x83xe2x80x83(equation 1)
When the non-applied state of the electric field or the strength of the applied electric field is sufficiently small as shown in FIG. 23(a), Eg of the semiconductor layer is determined according to the difference between an upper end Va of an energy level in a valence band and a lower end Ca of an energy level in a conduction band. If the absorption of the light by the semiconductor layer is discussed by band-to-band transition alone, then the light incident to the semiconductor layer transitions electrons in the valence band to the conduction band by its own energy, so that the light is absorbed into the semiconductor layer. Therefore, light of energy (wavelength longer than xcexg) lower than Eg is not substantially absorbed. However, when the electric field to be applied to the semiconductor layer is strengthened, respective degeneration of the energy levels are released so that they are split into a plurality of energy levels. This phenomenon is called xe2x80x9cStark effectxe2x80x9d. As a result of its splitting, the transition of electrons between the valence band and the conduction band due to the energy levels Va2 and Ca1 is newly added. Since an energy difference Egxe2x80x2 between the levels is smaller than Eg, the range of the wavelength of the light absorbed by the corresponding semiconductor layer is shifted to the long-wave side up to xcex1gxe2x80x2 equivalent to Egxe2x80x2.
On the other hand, when the degree of the absorption of light by the semiconductor layer depends on an absorption coefficient a and light having an intensity I0 enters into the semiconductor layer by a distance x, a light intensity I(x) attenuated by the absorption of the semiconductor layer is represented like the following equation:
I(x)=I0exp{xe2x88x92xcex1x}=I0exp{xe2x88x922xcfx89xcexax/c}xe2x80x83xe2x80x83(equation 2)
where xcfx89(sxe2x88x921) indicates an angular frequency (xcfx89=2xcfx80xcexd, xcexd: a light""s wave number), xcexa (dimensionless) indicates an attenuation coefficient (also called extinction coefficient), and c indicates the velocity of light in vacuum (about 3.0xc3x97108m/s), respectively. The extinction coefficient is a parameter indicative of an imaginary part of a refractive index (n+ixcexa=C/v, v: the velocity of light traveling or propagating in a medium) of the medium accompanied by light absorption.
The absorption coefficient xcex1 depends on the energy (hv) of light as shown in FIG. 23(b). The absorption coefficient a satisfies the relations in the following equation in a state free of the splitting of the above-described energy levels (it is illustrated as a curve a). Here, h indicates a Planck constant (about 6.63xc3x9710xe2x88x9234 Jxc2x7s).
xcex1xe2x88x9d (hxcexdxe2x88x92Eg)xc2xd/xcfx89xe2x80x83xe2x80x83(equation 3)
On the other hand, when the energy levels are split, the relationship between xcex1 and hxcexd is illustrated as a curve b in which the rise of the curve is shifted to Egxe2x80x2 lower in energy than Eg. Egxe2x80x2 is further shifted to the low energy side as the strength of an electric field to be applied increases.
Of the conventional optical modulators, the principle of the above-described light absorption has been generally used in one which utilizes a wave-guiding channel of each semiconductor layer. In this case, the wave guide layer for wave-guiding a light signal of wavelength xcexsig (energy Esig) is composed of a semiconductor material in which the forbidden band width Eg at the non-application of the electric field takes Eg xe2x89xa7Esig, and the forbidden band width of the wave guide layer or the cladding layer joined thereto is set smaller than Esig to absorb the light signal. As indicated by the curve b of FIG. 23(b), the absorption coefficient a at the application of the electric field on the side of low energy less than Eg is smaller than that on the side of high energy greater than Eg. However, the modulation performance sufficiently adoptable in the optical communication system has already been achieved.
The conventional optical modulator has been disclosed in, for example, Japanese Patent Application Laid-Open Nos. Hei 7-230066 and 8-86986. A technique related to this has been described even in U.S. Pat. No. 4,826,295. Further, a (monolithic) optical apparatus in which a semiconductor optical modulator is formed over the same substrate as a semiconductor laser light source, has been disclosed in, for example, Japanese Patent Application Laid-Open Nos. Hei 6-21578 and 9-181682. The light signal modulation technology described in these publications lies within the level of the practical use at present. It is reported that at the study level, a light signal modulation of 50 GHz at maximum is obtained by a technique related to the optical modulator described in Japanese Patent Application Laid-Open No. Hei 7-230066 (T. Ido, et al, xe2x80x9cELECTRONICS LETTERSxe2x80x9d, Vol. 31, No. 24 (1995), pp. 2124-2125). This record data has been put on the stage as a support of the implementation of the optical transmission at the transmission rate of 40 Gb/s. However, there is no report that this modulation frequency has been reproduced subsequently, to say nothing of the data for updating this record.
The present inventors have interpreted the cause by reference to FIG. 24 as follows. A conventional semiconductor optical modulator 200 is constructed such that a wave guide layer 203 and cladding layers 202 and 204 interposing it therebetween and having conduction types opposite to each other are stacked over a semiconductor substrate 201, and electrodes 205 and 206 are formed over upper and lower surfaces of a laminated structure including the semiconductor substrate so as to interpose the laminated structure therebetween. Namely, this laminated structure would become a diode having a pn type or pin type junction. Since a modulation electric field is applied to the electrodes provided at the upper and lower positions of the laminated structure in a direction opposite to the pn type or pin type junction, the semiconductor optical modulator would become one capacitive element (capacitor). The capacitance C thereof is given from permittivity ∈ of the entire laminated structure, the thickness d thereof in the laminated direction, and an electrode area S as follows:
C=∈S/dxe2x80x83xe2x80x83(equation 4)
The electric field is applied between the electrodes of the semiconductor optical modulator intermittently, e.g., in pulse form according to signal information to be transmitted. On the other hand, the capacitance C serves as the time constant with respect to a light signal modulation circuit including the corresponding optical modulator. Thus, when the modulation frequency increases, the influence of slowdown in the response of formation and erasure of the modulation electric field in the laminated structure becomes large so that the implementation of desired optical modulation falls into difficulties.
On the other hand, the structure of an optical device or element of a type wherein a semiconductor optical modulator is fabricated in a resonator of a semiconductor laser, has been disclosed in Japanese Patent Application Laid-Open No. Hei 6-169124. An outline of a cross section thereof extending in the direction of the resonator (i.e., in an oscillating direction of laser light) is shown in FIG. 25. According to the present optical device, a modulation layer 213, a second conductivity type second semiconductor layer 214 opposite to a first semiconductor layer, an active layer 215 (light emitting region), first conductivity type third and fourth semiconductor layers 216 and 217, and a second electrode 218 are laminated over the upper surface of the first conductivity type first semiconductor layer 212 in this order. Further, a voltage V1 is applied between the second semiconductor layer 214 and the fourth semiconductor layer 217 in pulse form, and a voltage V2 is applied between the first semiconductor layer 212 and the second semiconductor layer 214 in pulse form, respectively. V1 is applied in the forward direction with respect to the junction direction of the conductivity type in the laminated structure, whereas V2 is applied in the reverse direction with respect to it. The former controls a light-emission phenomenon developed in the active layer and the latter controls absorption of light produced from the active layer 215 into the modulation layer 213, respectively. A pulse phase control line (not shown) is provided between power supplies or sources 221 and 222 (current supply source for V1) for V1 and V2. Both the power supplies apply voltage pulses to each individual layer of the laminated structure at equal intervals and with a predetermined phase difference. Since the laser light intermittently generated by the application of the pulse voltage V1 (pulse current incident to it) is absorbed by the modulation layer according to the turning on or off of the pulse voltage V2 during the former or latter half of its generation time, the time-axial width of the pulsed laser light becomes short by an absorption time interval.
As described above, the method of intermittently supplying a laser driving current produced from each power supply and thereby modulating laser oscillations is called xe2x80x9cdirect modulation scheme or systemxe2x80x9d. In the direct modulation system, the response of the laser oscillations in pulse form to the voltage signal V1 (driving current) is placed under the influence of the life of a carrier related to an oscillated threshold current and a light-emission phenomenon of laser light, and the generation and extinction of the laser light are respectively delayed with respect to the turning on/off of the voltage signal (driving current) (this phenomenon is called xe2x80x9coscillation delayxe2x80x9d). An oscillation delay time interval is about 10xe2x88x929s (seconds) if described specifically. Therefore, when a serial type optical communication system is constructed by the direct modulation scheme, the limit of the transmission rate would become 1 giga-bit/s (109bit/s) or less.
A summary of the oscillation delay will be explained with reference to FIG. 8. Output delays of a light signal 61 with respect to the input (injection of driving current) of a voltage signal 60 are different from each other according to on/off intervals of the voltage signal as in the case of xcex94t1, xcex94t2, xcex94t3 and xcex94t4. This is because the oscillation delay time interval is taken to experience the effect of storing or accumulating the carriers injected into the active layer. Variations in delay time appear even in the delay of extinction of the light signal subsequent to the cutting-off of the voltage signal (driving current). Thus, even if the timing provided to recognize the signal on the light signal receiving side is shifted a predetermined time to prevent misrecognition of the transmission information on the light signal receiving side, a problem on its misrecognition cannot be essentially resolved. Further, even if so-called tailing of the laser light due to the delay of the extinction of the light signal with respect to the off time of the voltage signal is absorbed by the modulation layer disclosed in Japanese Patent Application Laid-Open No. Hei 6-169124, there is a possibility that this will be absorbed before the strength of the light signal with respect to the input of the voltage signal sufficiently rises. This falls short of solving the problem on the misrecognition of the transmission information.
2. Second Viewpoint (Multi-Waving of Light Signal):
In view of trends of the background art from the second viewpoint, for example, a wavelength variable type semiconductor laser is adopted in a signal light source to thereby set the wavelength of a light signal to a desired value according to the type of signal (information) to be transmitted. Most of optical fibers each constituting a signal transmission line absorb light incident thereto, and the amount of its absorption depends on the wavelength of light. Further, each optical fiber deforms profiles extending in time-axis and wavelength-axis directions of pulse light incident thereto and propagated therethrough by its own dispersion effect. Thus, when a plurality of light signals different in wavelength are transmitted through one optical fiber, there are demands for the restriction of the range of the wavelength of the signal light with respect to the light absorption and the proper setting of intervals between the wavelengths of the plurality of signal lights with respect to the deformation of each light pulse. The wavelength variable type semiconductor laser is a light source which meets these demands.
One of the wavelength variable type semiconductor lasers has been disclosed in Japanese Patent Application Laid-Open No. Sho 61-148890. An outline of a cross section thereof in the direction of a resonator thereof is shown in FIG. 26. This semiconductor laser is constructed in the following manner. An active layer 233 and crystal layers 234 (hereinafter abbreviated as xe2x80x9celectrooptic effect layersxe2x80x9d) each composed of a material showing an electrooptic effect are formed over the upper surface of an N type semiconductor substrate 232 with an electrode layer 231 provided on the lower surface thereof so that the former is interposed between the latter layers. A P type semiconductor layer 235 is further formed over the upper surface of the active layer, and an electrode layer 236 is formed over the upper surface of the P type semiconductor layer 235. On the other hand, a plurality of electrodes 237 (grid electrodes) are formed over the upper surface of the electrooptic effect layer 234 so as to be spaced away from each other and placed side by side in the resonator direction. The electrooptic effect layer 234 is formed of a material whose refractive index changes according to an electric field applied from each electrode 237 formed over the upper surface thereof and which is transparent to the light produced by the active layer.
When an electric field is formed between one set of adjacent grid electrodes 237, a refractive index change 238 occurs in an electric field application part of each electrooptic effect layer (this part will hereinafter be called xe2x80x9crefractive index change partxe2x80x9d). When each electrooptic effect layer 234 is formed of LiNbO3, a field strength necessary to form each refractive index change part 238 is about 1V/xcexcm. When the refractive index change parts 238 are formed at predetermined intervals a, a periodic refractive index distribution appears on the upper surface side of each electrooptic effect layer 234. This refractive index distribution selectively reflects light of wavelength xcex satisfying the following relation of light incident from the active layer to the electrooptic effect layer 234, toward the active layer.
a=Nxcex/2neffxe2x80x83xe2x80x83(equation 5)
where N indicates a natural number, and neff indicates an effective refractive index of each electrooptic effect layer 234. The periodic refractive index distribution functions as a diffraction grating. As viewed from the example shown in FIG. 26, the light reflected by the right electrooptic effect layer 234 is longer in wavelength than the light reflected by the left electrooptic effect layer 234. Although the respective periodic refractive index distributions of the left and right electrooptic effect layers are made different from each other in FIG. 26 for description, the left and right periodic refractive index distributions are normally formed equally, whereby induced emission occurs in the active layer with respect to the wavelengths reflected from the left and right electrooptic effect layers 234, thus oscillating or producing laser light of the wavelengths referred to above.
The refractive index change due to the field application at the semiconductor laser results from the electrooptic effect. As a material which makes up of the electrooptic effect layer 234, may be used a semiconductor material such as GaAs or the like in addition to KH2PO4, (Pb.La) (Zr.Ti)O3. It is necessary to cause the optical axis of each electrooptic effect layer, the direction (which is indicated by arrow at the lower portion of each refractive index change part) of an electric field formed by each grid electrode, and the direction (along the wave-guiding direction) of an electric field for inductively emitted light to coincide with each other for the purpose of improving the selectivity of the wavelength of each inductively emitted light according to the refractive index. Therefore, a potential difference must be provided between the grid electrodes 237 adjacent to each other as shown in FIG. 26. It is thus disadvantageous in terms of insulation between bonding wires for supplying voltages to the grid electrodes. Further, the present publication does not discuss in particular, the follow-up or response of laser light oscillations to the electric field applied to each grid electrode when the laser light is (intermittently) oscillated in pulse form by the semiconductor laser.
On the other hand, Japanese Patent Application Laid-Open No. Hei 7-326820 discloses a wavelength variable type semiconductor laser similar to the device structure disclosed in Japanese Patent Application Laid-Open No. Hei 6-169124. Its structure will be explained with reference to FIG. 25. Conductivity type of each semiconductor layer constituting a device or element roughly corresponds to that described in Japanese Patent Application Laid Open No. Hei 6-169124 but is different therefrom in that the above-described modulation layer is used as an active layer (light emitting region) and the above-described active layer is used as a tuning layer. The tuning layer and the active layer might be replaced in layout by each other. However, a description will be continuously made in the former case.
An electric field V1 applied from a second semiconductor layer 214 to a fourth semiconductor layer 217 via a tuning layer 215 by a power supply or source 221 in the direction opposite to a conductivity type sequence changes the refractive index of a semiconductor material which constitutes the tuning layer. On the other hand, a power supply 222 supplies a current as a current supply source under a voltage V2 applied from a first semiconductor layer 212 to the second semiconductor layer 214 via an active layer 213 in the forward direction with respect to the conductivity type sequence. In the semiconductor laser shown in FIG. 25, light of wavelength reflected by a diffraction grating 219 formed between a third semiconductor layer 216 and the fourth semiconductor layer 217, of lights produced by injecting a current into the active layer 213, is returned to the active layer 213 from which the light of the wavelength is inductively emitted. When an uneven cycle of the diffraction grating 219 is set as a, the light returned to the active layer 213 thereby has a wavelength xcex which satisfies the above (equation 5). At this time, the above-described neff is defined as an effective refractive index of the active layer 213. However, the value thereof is placed under the influence of the refractive index of each of the semiconductor layers interposing the active layer 213 therebetween. Therefore, a change in the refractive index of the tuning layer 215 determines the oscillation wavelength of the semiconductor laser as a change in neff.
However, the present publication does not discuss in particular, the follow-up or response of the change in neff to the applied field V1, i.e., the follow-up of the oscillation of laser light having a desired wavelength when the laser light is (intermittently) oscillated in pulse form by the semiconductor laser.
Thus, since the aforementioned wavelength variable type semiconductor laser has no attention to the oscillation of the laser light in pulse form under the control of the laser oscillation conditions, the use of the semiconductor laser in an optical communication system operated at a transmission rate of 1 Gb/s per wavelength needs to provide optical modulating means starting with the aforementioned semiconductor optical modulator at a stage subsequent to a resonator for laser oscillations.
It can be concluded that if the background art is generally summed up from the viewpoints 1 and 2, the generation of a light signal by each semiconductor optical modulator is essential to the speeding up of signal transmission and an increase in the capacity for the signal transmission in an optical communication system in terms of an improvement in the transmission rate. However, the semiconductor optical modulator is restricted in its modulation frequency in terms of its structure as discussed in the background art. Therefore, a problem remains with a view toward putting the optical communication system having a transmission rate of 20 Gb/s or more per wavelength into practical use. Thus, a wavelength division multiplexing (abbreviated as xe2x80x9cWDMxe2x80x9d) system for transmitting a plurality of light signals different in wavelength by one optical fiber should be inevitably adopted upon putting an optical communication system having a transmission rate of 20 Gb/s or more, particularly, a transmission rate of 40 Gb/s or more at which an effect is expected upon the speeding up of signal transmission and an increase in capacity therefor, into practical use.
Meanwhile, an example for simultaneously generating a plurality of lights having wavelengths by one resonator is not disclosed in the conventional semiconductor laser. Therefore, there has been a demand for the placement of semiconductor laser elements or devices on the signal transmitting side in parallel to a signal transmission line (optical fiber) according to wavelengths of light signals upon constructing the optical communication system using the wavelength division multiplexing system. It is important upon avoiding misrecognition of transmission information on the light signal receiving side that light signal transmitters (each including a semiconductor laser or a device with the semiconductor laser and an optical modulator placed in mixed form, and an optical module) provided all wavelengths are driven in synchronism with each other in the so-called parallel type optical communication system for dividing one transmission information into the light signals of plural wavelengths and transmitting them. However, the conventional optical communication system including the direct modulation scheme described in the background art has no attention to a reduction in variations in transmission delay time of light signals, which occur between the plurality of light signal transmitters. When a light signal of 1 Gb/s or more is transmitted from each of these light signal transmitters, it was difficult to provide synchronization between the light signal transmitters.
Further, in an optical communication system wherein each light signal transmitter is constructed by utilizing a semiconductor laser and an optical modulator in combination, laser light oscillated continuously from the semiconductor laser is modulated by the optical modulator according to transmission information (e.g., by performing switching between on and off) so that the transmission information is converted to a light signal. Namely, it is necessary to always supply a current of a so-called oscillation threshold or more necessary for the oscillation of the laser light to the semiconductor laser and apply a voltage corresponding to the current thereto. Therefore, the light signal transmitter raises the temperature of its own operating environment according to power to be used up or consumed. A temperature range of the operating environment of the light signal transmitter is maintained at a temperature of from xe2x88x9240xc2x0 C. to +70xc2x0 C. However, the wavelength of the laser light produced from the semiconductor laser slightly varies according to the temperature even in the case of this temperature range. Such a wavelength variation brings about misrecognition of transmission information on the receiving side in an optical communication system for identifying the transmission information by the wavelength. The wavelength division multiplexing type optical communication system will mistake the light signal of wavelength to be recognized. Most of the present light signal transmitters are respectively provided with temperature control means such as Peltier elements or the like to generate light signals stably with respect to a change in temperature. However, in a terminal (such as the ONU or the like) on the user side, which is hard to become careful of the environment, particularly, temperature management used for the light signal transmitter, the operating environment temperature will exceed the temperature range even in the case of the light signal transmitter added with the temperature control means. Even in the case where the temperature is simply raised by 10xc2x0 C. from an upper limit value of +70xc2x0 C. to +80xc2x0 C., the rise in the environment temperature greatly fails to stabilize the wavelength of the light signal of the light signal transmitter.
An object of the present invention is to improve a transmission rate of a light signal employed in the optical communication system (also called xe2x80x9coptical transmission systemxe2x80x9d). In order to achieve this object, one of the present inventions constructs or constitutes an optical communication apparatus capable of transmitting one type of light signal (signal light having one wavelength) at a transmission rate of 20 Gb/s more with satisfactory reproducibility (first invention). The first invention also provides the construction of an optical communication system which performs high-speed and high-capacity information transmission without having to use the wavelength division multiplexing system. Further, in order to achieve the above object, another one of the present inventions constitutes an optical communication apparatus suitable for providing synchronization between a plurality of types of light signals different in wavelength in an optical communication system for dividing transmission information into the plurality of types of light signals (signal lights of plural wavelengths) and transmitting the respective light signals at a transmission rate of 1 Gb/s or more (second invention).
Another object of the present invention is to improve the reliability of signal transmission in an optical communication system from the viewpoint of the wavelength of a light signal. In order to achieve the above object, the present invention constructs an optical communication apparatus and an optical communication system both capable of sufficiently controlling instability of a signal wavelength due to a rise in temperature on the light signal transmission side (third invention).
In any of the above-described first through third of the present inventions, laser light capable of greatly setting power inputted to an optical fiber corresponding to a signal transmission line is used as a light signal. The concept common to the respective inventions is to inductively emit signal light from each light signal transmitter, i.e., intermittently performing so-called laser oscillations according to a transmission signal. The concept of execution of the induced emission of the signal light from the light signal transmitter according to the transmission signal inputted thereto is similar to the technique described in Japanese Patent Application Laid-Open No. Hei 6-169124. However, the present invention is different from it in that the direct modulation scheme or system for modulating the current to be supplied to the laser oscillation unit is not adopted. In the present invention, (1) light is produced by spontaneous emission, i.e., light emission based on the so-called principle of operation of a light emitting diode, (2) a desired wavelength component (signal wavelength) of this light is supplied to a resonator for oscillating or producing laser light or a part (hereinafter called xe2x80x9claser light producing unitxe2x80x9d) corresponding to it according to a transmission signal as so-called pumping light (excitation light), and (3) induced emission based on the desired wavelength component is produced by the resonator.
A summary of the aforementioned phenomenon of spontaneous emission will now be explained by an example of a light emitting diode (LED). The light emitting diode is comprised of a laminated structure of semiconductors each composed of a p-n junction or a p-i-n junction. The emission phenomenon occurs due to the diffusion of electrons and positive holes at the time that the voltage is applied to the conductivity type of junction referred to above in the forward direction, and their coupling. Therefore, the density J[A/m2] of the total current in the forward direction (moving from a p type layer to an n type layer) which flows through the junction, is defined by the following equations:
J=J0{exp(qV/kT)xe2x88x921}xe2x80x83xe2x80x83(equation 6)
J0=qDhpn0/Lh+qDenp0/Lexe2x80x83xe2x80x83(equation 7)
xe2x80x83Le=(Dexcfx84e)xc2xdxe2x80x83xe2x80x83(equation 8)
where q indicates an electrical change of an electron (1.6022xc3x9710xe2x88x9219 C), V indicates a potential difference in the forward direction, k indicates a Boltzmann constant (1.381xc3x9710xe2x88x9223J/K), T indicates the temperature [K], and np0 ad Pn0 respectively indicate densities [mxe2x88x923] of an electron in a p region and a positive hole (so-called minority carrier) in an n region in a thermal equilibrium state. Further, Le, De and xcfx84e respectively indicate a diffusion distance [m], a diffusion coefficient [m2/s] and a life [s] of an electron in the p region. Those whose suffix letters e are replaced by n, respectively indicate a diffusion distance, a diffusion coefficient and a life of a positive hole in the n region. Further, if the life xcfx84r[s] of recombination accompanied by light emission based on the electron and positive hole is constant regardless of the location, then the number of photons Np generated from the light emitting diode and its internal quantum efficient xcex7 are respectively given by the following equations:
Np=np0Le{exp(qV/Kt)xe2x88x921}/xcfx84rxe2x80x83xe2x80x83(equation 9)
xcex7=qNp/Jxe2x80x83xe2x80x83(equation 10)
In a normal semiconductor laser (laser diode), a resonator for oscillating or producing laser light is made up of a semiconductor material and a region (active region) composed of an active medium is provided thereinside. The semiconductor laser is identical in function to a light emitting diode taken in a broad sense in that the aforementioned light emission phenomenon is produced by the injection of the current into the corresponding active region and the produced light is distributed over a certain wavelength range, but is different therefrom in that the above-described resonator is constructed so that a specific wavelength component of produced light is returned to an active region to thereby cause an induced emission phenomenon in the active region (e.g., a Fabry-Perot type structure interposing an active medium between reflecting mirrors is adopted). Namely, the semiconductor laser generates excitation light necessary for the induced emission by light produced inside its own resonator. Further, a current of a given value or more must be injected into the active region to obtain the excitation light necessary for the induced emission (this current value is called xe2x80x9cthreshold currentxe2x80x9d for the oscillation of laser light). It is necessary to apply an electric field of a voltage or more equivalent to the threshold current value to the semiconductor laser according to a current-voltage characteristic at the oscillation of laser light inherent in the semiconductor laser upon the injection of the current into the active region. It is needless to say that no laser light is produced under the condition in which the current injected into the active region of the semiconductor laser falls below the threshold. Further, the emission of the spontaneously-emitted light produced inside the resonator to the outside of the resonator is also very weak. It is therefore essential that the current greater than or equal to the threshold is supplied to the active region upon driving the semiconductor laser. Correspondingly, a consumption current increases and the peripheral temperature of the semiconductor laser also rises with its increase. In an optical communication system for transmitting a light signal at a transmission rate of 1 Gb/s or more, laser light normally oscillated from a semiconductor laser is intermittently cut off by field absorption or the like according to a transmission signal through the use of an optical modulator provided at a stage (on the signal transmission line side) subsequent to the semiconductor laser, thereby generating a light signal. Therefore, about the half of the current injected into the semiconductor laser is discarded by the cutting off of the laser light by the optical modulator.
On the other hand, the optical communication apparatus (signal transmitting terminal) is identical to the conventional semiconductor laser in that light used as a species for laser oscillations, i.e., photons are generated by the spontaneous emission through the use of a semiconductor light emitting element or the like, but is different therefrom in the principle that a photon having specific energy (wavelength) is supplied to a laser light generating unit as excitation light for induced emission. In the conventional light signal transmitter comprised of the combination of the semiconductor laser and the optical modulator as described above, the oscillations of the laser light by the induced emission and the generation of the light signal according to the transmission information are separately performed by the former and latter. This technique idea is identical to the semiconductor optical device disclosed in Japanese Patent Application Laid-Open Nos. Hei 6-21578 and 9-181682 wherein the two are monolithically formed. Further, it is inferred that the intention for separating both functions be included in a structure wherein a diffraction grating (having the function of selecting an oscillation wavelength) provided in a DFB-LD unit (laser light generating unit) of each of the devices disclosed in these publications is prevented from extending to an EA-MOD unit (optical modulator). In contrast to the conventional optical communication system for modulating the intensity of the thus inductively-emitted laser light according to the transmission information, the present invention has a great feature in that the intensity or wavelength of the excitation light related to the induced emission is modulated. In other words, the optical communication apparatus or system according to the present invention is constructed so as to change the induced emission condition of the laser light according to the transmission information.
Meanwhile, the optical communication apparatus and system according to the present invention need to ensure the transmission rate of the light signal at 1 Gb/s or more at minimum. Namely, the present invention needs to modulate the intensity or wavelength of the excitation light supplied to the laser light generating unit in a cycle of 10xe2x88x929s (seconds) or less. It is needless to say that this demand cannot satisfy the induced emission described in the background art by the injected current-based direct modulation scheme. It is also difficult to satisfy this demand even by a distributed Blagg reflection type (DBR type) resonator or a distributed feedback type (DFB type) resonator utilizing the generation and extinction of a diffraction grating by the electrooptic effect disclosed in Japanese Patent Application Laid-Open No. Sho 61-148890, i.e., Pockels effect, Kerr effect or the like. However, the present inventors have conceived a new method of modulating induced emission condition, which is different in principle from the direct modulation scheme and the electrooptic effect, and have found that the intensity or wavelength of the excitation light supplied to the laser light generating unit can be modulated even in a short cycle of 10xe2x88x929s or less. The present inventors will first introduce the concept of an optical communication apparatus (signal transmitting terminal) according to the present invention by reference to one example shown in FIG. 1 and will next explain the summary of the principle of a new method of modulating an induced emission condition for signal light, according to the present invention in association with the functions of the optical communication apparatus and optical communication system.
An optical communication apparatus 10 used as a light signal transmission apparatus shown in FIG. 1(a) is separated into three sections of a light signal transmitter or transmission unit 1, a noise eliminator 2 and a light signal amplifier unit 3. A light signal produced from the light signal transmission unit 1 passes through the noise eliminator 2 and the light signal amplifier unit 3 via an optical fiber 20, followed by transmission to a signal transmission line (optical fiber). The light signal transmission unit 1 includes, as principal component requirements, a signal light generating unit (laser light generating unit) 15 including a light emitting portion and an induced emission condition controller (both not shown), a power supply 11 for supplying a current necessary for spontaneous emission at the light emitting portion, and a signal source 12 for inputting transmission information to the induced emission condition controller. The signal source 12 receives transmission information sent from users 12a and 12b therein and sends an electric signal to the induced emission condition controller so as to modulate an induced emission condition of the light signal transmission unit 1 according to the transmission information. The noise eliminator 2 is provided to cut off light incident from the signal transmission line to the optical communication apparatus 10 and prevent spontaneously emitted light produced in the light signal transmission unit 1 from leaking to the signal transmission line. For the purpose of the former, an optical isolator is constructed which comprises a polarized-plane rotator using the Faraday effect. The optical isolator comprises a polarizer 21 in which the direction of rotation of a polarized plane thereof with respect to a magnetic field is 0xc2x0, a magneto-optic rotator (rod-like lens composed of a material such as Y3Fe5O12 (YIG) or the like) in which the direction of rotation of its polarized plane is 45xc2x0, and an analyzer 23 in which the direction of rotation of its polarized plane is 45xc2x0. For the purpose of the latter, a metal film such as nickel (Ni) or chromium (Cr) or the like is evaporated on the light signal transmission unit 1 side of the magneto-optic rotator 22 to form an optical attenuator 24. The optical attenuator 24 sets the material of the metal film and the evaporated thickness thereof according to the extent that spontaneously emitted light formed as noise is attenuated relative to a signal of laser light. Although the optical attenuator 24 is not fabricated and built in the optical isolator, any forming position may be used if placed over an optical path which connects the light signal transmission unit 1 and the light signal amplifier unit 3 to each other. The metal film may be formed on, for example, a lens or the like optically coupled to the optical isolator. The light signal amplifier unit 3 is provided to avoid a loss of a light signal intensity within the signal transmission line before the light signal (laser light) is launched into the signal transmission line. In the example shown in FIG. 1(a), a light source 33 for generating excitation light for exciting an active medium placed within the fiber, a power supply 31 for operating the light source, and a directional coupler 36 for combining the signal light sent from the light signal transmission unit 1 with the excitation light and sending the combined one to an optical fiber amplifier 35 are placed in a preceding stage (on the light signal transmission unit 1 side) with the optical fiber amplifier 35 as the center, and an optical detector 34 for monitoring an amplified state of the fiber amplifier, a power supply 32 for activating the optical detector 34 and a directional coupler 37 for taking out light to be monitored from the optical fiber 20 are placed in a subsequent stage (on the signal transmission line side), respectively.
The modulation of the induced emission condition for the light signal employed in the optical communication apparatus of the present invention illustrated by way of example in FIG. 1(a) is based on the input of the electric signal from the signal source 12 to the induced emission condition controller included in the signal light generating unit 15 of the light signal transmission unit 1. FIG. 2 shows two types of specific examples of the signal light generating unit 15 as cross sections taken along the oscillating direction of the laser light. Both are semiconductor devices each constructed such that an n type first semiconductor layer 102, an active region 103 composed of a semiconductor material, which is smaller in forbidden band width than the first semiconductor layer, a p type second semiconductor layer 104 larger in forbidden band width than the active region, and a p type third semiconductor layer 105 different in refractive index from the second semiconductor layer are stacked over an n type semiconductor substrate 101 in this order. In both the semiconductor devices, electrode layers 106 are respectively formed over the lower surfaces of the semiconductor substrates 101. Requirements satisfied to allow the semiconductor layer 103 to function as the active region are to combine a forbidden band width small as compared with the semiconductor layers 102 and 104 joined to each other as viewed in the vertical direction with a refractive index large as compared therewith. The active region 103 might be constructed by a so-called quantum well type structure in which a thin semiconductor layer called quantum well layer is interposed between barrier layers larger in forbidden band width than the semiconductor layer. In this case, however, it is necessary to set the refractive indexes of the quantum well layer and barrier layers so as to be larger than those for the semiconductor layers 102 and 104.
In a semiconductor device corresponding to a signal light generating unit 15 disclosed in FIG. 2(a), a stripe-like electrode layer 107 extending in the oscillating direction of laser light is provided over the third semiconductor layer 105. A potential difference of xcex94V1 is set substantially constant between the electrode layers 107 and 106 by a power supply 11 so that the current is supplied therebetween. Spontaneous emission occurs in the active region 103 owing to the current supply. A plurality of electrode layers 109 are formed over the electrode layer 107 with an insulating layer 108 interposed therebetween so as to be spaced away from each other in the oscillating direction of the laser light. The width of each electrode layer 109, which crosses the oscillating direction of the laser light, is set larger than a stripe width of the electrode layer 107. Further, the electrode layer 106 and the electrode layers 109 are constructed so as to be opposed to each other on both sides of the stripe of the electrode layer 107. An electric field having a potential difference of xcex94V2 is applied between the electrode layer 106 and each electrode layer 109 by a power supply 12. Large and small changes in the potential difference are repeated according to transmission information. In regions 110 developed in the second and third semiconductor layers 104 and 105 so as to correspond to the electrode layers 109, the densities of two-stage patterns thereof depend on the magnitude of a change in refractive index in a refractive index change region in which prosperity and decay are repeated with the changes in the potential difference of xcex94V2. Part of light spontaneously emitted from the active region 103 leaks to the second semiconductor layer 104 adjacent to the active region. Thus, the light having the wavelength satisfying the condition given in the equation 5 described in the background art with respect to each interval a between the refractive index change regions 110 developed in the semiconductor layer so as to be spaced away from each other in the oscillating direction of the laser light is returned to the active region 103, where the induced emission of the light having the wavelength is produced. Namely, in the signal light generating unit 15 illustrated in FIG. 2(a) by way of example, the electrode layers 106 and 107 and the active layer 103 interposed therebetween correspond to a light emitting portion, and the electrode layers 106 and 109 and the active region 103 interposed therebetween correspond to an induced emission condition controller or control portion, respectively.
Further, a semiconductor device used as a signal light generating unit 15 disclosed in FIG. 2(b) is common to the semiconductor device shown in FIG. 2(a) in principal component requirements but is different therefrom in that a light emitting portion 151 including the active region 103 interposed between the electrode layers 106 and 107, and an induced emission condition controller or control portion 152 including the active region 103 interposed between the electrode layers 106 and 109 are respectively formed so as to be separated into the right half and the left half from each other, and a dielectric film 153 for setting a reflection power of a right end surface lager than that of a left end surface is formed over the right end surface. In the light emitting portion 151, a semiconductor layer, so-called contact layer 111 smaller in forbidden band width than a third semiconductor layer or higher in impurity density than that is formed between the third semiconductor layer 105 and the electrode layer 107. The electrode layer 109 formed in the induced emission condition control portion 152 is not divided along an oscillating direction of laser light. A diffraction grating (irregularities developed in a junction interface) is formed between a second semiconductor layer 104 and the third semiconductor layer 105 located below the electrode layer 109. A high-resistance region 154 formed by Zn ion-implantation or its diffusion or the like is formed between the light emitting portion 151 and the induced emission condition control portion 152. However, this region can be omitted depending on specification conditions of the semiconductor device.
The semiconductor device disclosed in FIG. 2(b) constitutes a Fabry-P3rot type resonator by interposing the light emitting portion 151 between the induced emission condition control portion 152 and the dielectric film 153. Laser light developed by the resonator has a wavelength reflected toward the light emitting portion 151 by the diffraction grating 112 formed in the induced emission condition control portion 152. However, the reflected wavelength changes according to an electric field applied between the electrode layers 106 and 109 of the induced emission condition controller 152.
A method of modulating an induced emission condition for a light signal, according to the present invention will be explained with the above-described semiconductor devices as examples. The method according to the present invention basically makes use of a change in refractive index, which occurs due to the application of the electric field to a crystal of a semiconductor material. This phenomenon is based on the Stark effect described in the section of the background art or a Franz-Keldysh effect in which light incident to the crystal is concerned. These effects, i.e., the former and latter are respectively considered to occur at 107eV/cm or more and at 106eV/cm or less according to the applied strength of electric field. Each semiconductor optical modulator described in the background art makes use of the action of absorption of signal light by these effects, a so-called field absorption effect. In each of the optical signal transmitters in which the semiconductor lasers and optical modulators disclosed in Japanese Patent Application Laid-Open Nos. Hei 6-21578 and 9-181682 are monolithically formed, practically, a current is injected into the semiconductor laser by the application of an electric field corresponding to a voltage of from 1.2V to 1.3V thereto to thereby generate laser light, whereas an electric field corresponding to a voltage amplitude of from 2V to 3V is applied to the optical modulator to modulate the laser light. In the present invention in contrast to this, a voltage signal having a vibrational waveform corresponding to transmission information is applied to the electrode layer constituting the induced emission condition controller of the semiconductor device with the voltage amplitude smaller than the electric field applied to the light emitting portion (while there is also an example in which the relationship between the voltage amplitude of the signal and the field strength is not inevitable in a structure of part of a light signal transmitter to be described later, this relationship is inevitable upon implementation of many optical communication systems according to the present invention). Namely, the voltage signal applied to the induced emission condition controller according to the present invention is much different from that described in the background art in that it is not aimed to modulate the absorption of the laser light.
The present inventors have paid attention to the fact that when an electric field is applied to a semiconductor material in a short time width of 10xe2x88x929s or less, the electric field acts within the semiconductor material as an electromagnetic wave, i.e., in a manner similar to light. A vibrational electric field E defined by an angular frequency xcfx89 and an amplitude E0, which is given by the following equation, is applied to a material which exhibits a polarization response of a dielectric or the like:
E=E0exp{ixcfx89t}xe2x80x83xe2x80x83(equation 11)
At this time, an electrical displacement D developed in the material with a phase shift of xcex4 is given by the following equation:
D=D0exp{i(xcfx89txe2x88x92xcex4)}xe2x80x83xe2x80x83(equation 12)
The permittivity ∈ of the material becomes a function of an angular frequency xcfx89 of an applied field as represented by the following equation (this will be called xe2x80x9ccomplex permittivityxe2x80x9d):
∈(xcfx89)=D/E=∈xe2x80x2(xcfx89)xe2x88x92i∈xe2x80x3(xcfx89)xe2x80x83xe2x80x83(equation 13)
In the equation 13, ∈xe2x80x2(xcfx89) of the real part indicates the storage of the applied electric field, and ∈xe2x80x3(xcfx89) of the imaginary part indicates a loss developed as thermal energy of the applied electric field by dispersion. Although a detailed description is omitted, when an electric field E vibrated in the form of a sine wave is applied in pulse form, a frequency response function is determined relative to a change in dielectric constant ∈p (pulse response function) responsive to it as given by the following equation:
∈(xcfx89)=∫0∞∈p(t)xc2x7exp{i∈t}dtxe2x80x83xe2x80x83(equation 14)
On the other hand, when an electromagnetic wave propagated through the material at a velocity v is absorbed in the material, the material exhibits a complex refractive index n* defined by the following equation:
n*=nxe2x88x92ixcexa=c/vxe2x80x83xe2x80x83(equation 15)
where the definition of xcexa and c is identical to one expressed in the equation 2 in the section of the background art. The following relation is established between the complex permittivity ∈(xcfx89) given in the equation 13 and the above complex permittivity:
(∈(xcfx89))2=n*xe2x80x83xe2x80x83(equation 16)
Thus, the following relations are respectively established between a refractive index n and an extinction coefficient xcexa in a state free of the absorption of the material and between the above
∈xe2x80x2 (xcfx89) and ∈xe2x80x3 (xcfx89)
∈xe2x80x2(xcfx89)=n2xe2x88x92xcexa2xe2x80x83xe2x80x83(equation 17)
∈xe2x80x3(xcfx89)=2nxcexaxe2x80x83xe2x80x83(equation 18)
A supplemental description will be made of I(x) indicative of the attenuation of the intensity of light in the medium in the equation 2 introduced in the background art. The value of the I(x) is defined as given by the following equation with respect to a field strength E(x) of light propagated through the medium:
xe2x80x83I(x)=|E(x)|2xe2x80x83xe2x80x83(equation 19)
If this is replaced by the propagation of the electromagnetic wave in the material, then the above I(x) is equivalent to attenuation along an electric field applying direction (x direction) of a potential difference. Therefore, the above-described equation 2 can be rearranged like the following equation:
V(x)=V2exp{xc3x97xcex1x}=V2exp{xe2x88x922xcfx89xcexax/c}xe2x80x83xe2x80x83(equation 20)
Namely, there is a possibility that when a vibrational electric field is applied to a material showing a polarization effect, the potential difference developed in the material by the electric field will show a steep change (attenuation) based on the equation 20, rather than a linear change as the electric field propagates through the material as an electromagnetic wave. It is further apparent from the equations 17 and 18 that a parameter xcexa indicative of the possibility of this attenuation depends on the values of the real and imaginary parts of the complex refractive index of the material, which are determined according to how to apply the angular frequency xcfx89 and pulse of the vibrational electric field, for example.
The present inventors have found out the attenuation of the potential difference expressed in the equation 20 when an electric field having a time width of 10xe2x88x929s or less is applied to a semiconductor material with limited voltage amplitude thereof. This attenuation phenomenon will be typically explained with reference to FIG. 3. The right side of FIG. 3 shows a cross-section of an induced emission condition controller of an optical communication apparatus according to the present invention, which is orthogonal to the direction of propagation of a light signal. For simplification of illustration, the semiconductor substrate and third semiconductor layer are omitted from FIG. 2. Further, an electrode layer 109 is directly formed over a second semiconductor layer 104 by the Schottky junction with no insulating layer interposed therebetween. The left side of FIG. 3 shows attenuation of a potential difference developed in a laminating direction of the induced emission condition controller due to an electric field applied between the electrode layers 106 and 109 in the forward direction in such a manner that values normalized at V2 are associated with the center of a refractive index change portion developed in a laminating direction of the right side of FIG. 3. Potentials of 0V and +1V are alternately applied to the electrode layer 109 at intervals of 1xc3x9710xe2x88x929s in association with the electrode layer 106 which is at a ground potential at all times. When an electric field of +1V is applied to the electrode layer 109 for 1xc3x9710xe2x88x929s, the potential at the second semiconductor layer is reduced to +1V at a junction interface with the electrode layer 109, +0.1V at the bottom of a refractive index change region 110a, +0.01V at the bottom of a refractive index change region 110b, and +0.001V at the bottom of a refractive index change region 110c, thereby reaching a junction interface with the electrode layer 106 of the first semiconductor layer of xc2x10V. The degree of a change in the refractive index in a refractive index change region 110 is abruptly reduced according to the reduction in the potential difference and depends on the material for the second semiconductor layer. However, this can be roughly neglected at the bottom of the refractive index change region 110c. Further, the degree of the reduction in the potential difference depends not only on the material for the second semiconductor layer but also on its structure. When, for example, a quantum well structure, i.e., a structure in which materials different in forbidden band width from each other are alternately laminated on one another in a thickness range of from above 2 nm to below 15 nm, is introduced in the vicinity of the bottom of the refractive index change region 110b, the refractive index greatly changes in this region and the extension of the refractive index change region 110c to an active region 103 is reduced shallow (this results from the quantum containment Stark effect).
In the semiconductor device shown in FIG. 2(a), part of the light spontaneously emitted from the active region 103 leaks to the first and second semiconductor layers 102 and 104 adjacent to each other. When refractive index differences are now formed in the second semiconductor layer 104 in cycles of intervals a by the refractive index change portions developed by the application of the electric field, the light having the wavelength component xcex satisfying the relation expressed in the equation 5 described in the background art is reflected toward the active region 103. Carriers, i.e., electrons and positive holes exist in the active region 103 due to the current injection but a carrier distribution of the active region 103 is reversed by the excitation developed due to the incidence of the reflected wavelength xcex. Thereafter, laser light of wavelength xcex is emitted by the recombination of the excited carriers. The above-described phenomenon shows a response excellent in prosperity and decay of each periodic refractive index difference developed by a very short field applying time of 10xe2x88x929s or less alone. Even if the field applying time is reduced up to 10xe2x88x9211s equivalent to a limited speed of an electrical circuit, for inputting transmission information to a light signal transmitter, i.e., about 100 GHz, a light signal of short pulses equivalent to this can be generated. Thus, the system of generating the light signal, which has been described with the semiconductor device shown in FIG. 2(a) as the example, will be called xe2x80x9cinduced emission modulation scheme or systemxe2x80x9d for convenience.
In the semiconductor device shown in FIG. 2(b) on the other hand, the wavelength reflected by the diffraction grating 112 formed between the second and third semiconductor layers 104 and 105 is modulated while the refractive index of the second semiconductor layer 104 is being changed. The semiconductor device is identical in operational function even to a structure shown in FIG. 4 wherein an induced emission condition control portion 152 and a light emitting portion 151 are separated from each other, and the latter is used as a light emitting diode of a surface emission type (having a form for emitting light in a laminating direction of a semiconductor layer). The light emitting diode shown on the right side of FIG. 4 is constructed by laminating on one another, a reflecting mirror layer 161 formed by alternately stacking n type semiconductor layers different in refractive index over an n type semiconductor substrate 101, an n type first semiconductor layer 162, an active layer (active region) 163 composed of a semiconductor material, which is smaller in forbidden band width than the first semiconductor layer, a p type second semiconductor layer 164 larger than the active layer in forbidden band width, and a third semiconductor layer 165 smaller in forbidden band width than the second semiconductor layer or higher in contained impurity density than that. An opening for allowing light spontaneously emitted from the active layer 163 to pass therethrough is provided in the center of an electrode layer 107 ohmic-junctioned with the third semiconductor layer.
In either of the structures shown in FIG. 2(b) and FIG. 4, the light spontaneously emitted from each of the active regions 103 and 163 of the light emitting portions 151 is wave-guided to the active region 103 in the induced emission condition control portion 152. However, part of the light is wave-guided to the first and second semiconductor layers 102 and 104 joined to the active region so as to leak thereto. Therefore, an effective refractive index neff at the induced emission condition control portion 152 is not univocally determined at a refractive index n1 and is affected by refractive indexes n3 and n2 of the first and second semiconductor layers 102 and 104. As is apparent from this, the reflected wavelength xcex determined by the equation 5 changes according to the presence or absence of the application of the electric field where a refractive index change region extending along the diffraction grating 112 is formed in at least part of the second semiconductor layer 104 by the application of the electric field. The diffraction grating 112 of the induced emission condition controller 152 serves as a Blagg reflecting mirror, sends light of wavelength xcex satisfying the condition expressed in the equation 5 to the light emitting portion 151 as excitation light, and urges the active regions 103 and 163 to inductively emit laser light of wavelength xcex. The light of wavelength xcex transmitted through the light emitting portion is returned to the light emitting portion again by the reflection of the dielectric film 153 or the reflecting mirror layer 161, where the laser light of wavelength xcex is inductively emitted. Namely, the semiconductor devices disclosed in FIG. 2(b) and FIG. 4 respectively have the crossed or combined function of the Fabry-P3rot type resonator and distributed Blagg reflection type resonator. When an optical communication apparatus is constructed of this device, the wavelength of a light signal is set to either a reflected wavelength at the application of the electric field or a reflected wavelength at the non-application thereof, and light having a desired wavelength is intermittently emitted according to transmission information by the application of the electric field to the induced emission condition controller 152. The response of the induced emission to the application of the electric field to each of the induced emission condition control portions 152 in these light signal transmitter configurations is identical to that in the semiconductor device shown in FIG. 2(a). Thus, the system for generating the light signal, which has been described by taking each of the semiconductor devices shown in FIG. 2(b) and FIG. 4 as an example, will be called xe2x80x9cwavelength modulation scheme or systemxe2x80x9d for convenience.
Examples of distributed Blagg reflecting mirror (DBR) type optical elements or devices wherein induced emission condition control portions 152 are respectively separated from light emitting portions as in the example shown in FIG. 4, are shown in FIG. 5. Of semiconductor layers constituting these, ones common in specification to the semiconductor device shown in FIG. 2 are respectively identified by the same reference numerals. Further, each layer 113 corresponding to an active region is changed in reference numeral as a wave-guiding layer to define that this layer has only a light wave-guiding function and is not related to light emission. While the optical device shown in FIG. 5(a) corresponds to the configuration of the induced emission control controller 152 shown in FIG. 2(a), a quantum well structure 114 is formed between second and third semiconductor layers 104 and 105. Further, the optical device shown in FIG. 5(b) corresponds to the configuration of the induced emission condition control portion 152 shown in FIG. 2(b). However, semiconductor regions 115 different in refractive index from a second semiconductor layer are embedded in the second semiconductor layer so as to be spaced away from each other in a propagating direction of a light signal, thereby forming a diffraction grating. Incidentally, the semiconductor regions 115 extend in bar form so as to intersect the propagating direction of the light signal, i.e., take the direction orthogonal to the sheet.
An example in which a signal light generating unit 15 is constructed by an optical fiber constructed inclusive of an optical active material, is shown in FIG. 1(b) as one example of a light signal transmitter utilizing these optical devices (DBR type optical devices). The optical fiber is constructed so as to cover the periphery of a core layer with a cladding layer lower in refractive index than it (this is called xe2x80x9cbare fiberxe2x80x9d). Further, the optical fiber is constructed so as to cover the periphery of the cladding layer with a covering material according to a use environment. The optical fiber may be composed of quartz as a principal component, or composed of a polymethyl methacrylate (PMMA) resin, for example. However, the former is used as a signal transmission line for optical communications. A small quantity of additives are suitably introduced in the former to control or adjust the refractive indexes of the core layer and the cladding layer. As materials for increasing the refractive index, P2O5, GeO2, Al2O3 and TiO2 are used, and as materials for decreasing the refractive index, B2O3 and F are used, respectively. The optical fiber used as the signal light generating unit 15 shown in FIG. 1(b) is constructed so as to contain an active material such as Er or the like in the core layer. The optical fiber has the property of causing induced emission therewithin when the inside thereof is irradiated with excitation light. When light spontaneously emitted from a light emitting diode 13 is launched into the optical fiber, the active medium provided within the optical fiber is slightly excited but do not produce induced emission. However, an optical device or element 14 shown in FIG. 5 is placed in a stage subsequent to the optical fiber. Further, an electric field is applied to the optical element shown in FIG. 5(a), and an electric field is applied or non-applied to the optical element shown in FIG. 5(b) according to the reflected wavelength of the diffraction grating, whereby the optical element 14 is caused to reflect light of signal wavelength toward the optical fiber. Since, for example, lights having coincident phases of ones weaker in the intensity of the reflected light than the semiconductor devices shown in FIG. 2 can be supplied to the optical fiber, the induced emission of laser light of signal wavelength by the corresponding optical fiber is produced, whereby signal light can be emitted. As is seen from this example, various configurations can be implemented by combinations of the light signal transmitters using the optical devices shown in FIG. 5 with other optical devices.
When an optical communication apparatus (transmitting terminal) or an optical communication system is now constructed by the light signal transmitters of wavelength modulation schemes illustrated in FIG. 2(b), FIG. 4 and FIG. 5(b) by way of example, it is desirable that laser light of wavelength, which does not contribute to information transmission, is set so as not to reach a transmission line (optical fiber). It is therefore recommended that a band pass filter 4 like the Fabry-p3rot etalon is provided between a light signal transmitter 1 constructed inclusive of such a semiconductor device and a transmission line as shown in FIG. 6(a). FIG. 6(b) is a diagram showing one example of the Fabry-p3rot etalon (this will be described in details later).
The functions for the light signal transmission according to the present invention have been described above. However, if the features of the present invention are summarized in association with the above-described first to third inventions adapted to the objects of the present invention, then they are as follows:
An optical communication apparatus according to the present invention comprises means for generating light signals according to induced emission, and light intensity attenuating means optically coupled to the light signal generating means. The light intensity attenuating means is optically coupled to a light signal transmission line. The light signal generating means has a light emitting portion for generating light according to the injection of a current therein, an induced emission unit for inductively emitting each light signal, and an excitation light generating unit for producing the induced emission. The excitation light generating unit supplies light (excitation light) of a specific wavelength component, which is included in the light produced in the light emitting portion according to the application of an electric field corresponding to transmission information, to the induced emission region. This construction is particularly suitable for an optical communication apparatus using the induced emission modulation scheme in that light of a noise component produced by spontaneous emission (light-emission mechanisms characterized by the above-described equations 6 to 10) based on the injection of a current into the light emitting portion is attenuated by the light intensity attenuating means and desired signal light (laser light) is selectively extracted. The light emitting portion, the induced emission unit and the excitation light generating unit included in the light signal generating means are not limited to their layouts if they fall within the range satisfying the above-described functions because there is an example in which they are built in the same semiconductor device as shown in FIG. 2(a) or an example in which they are separated from each other as shown in FIG. 1(b).
The light signal generating means according to the present invention does not supply a current greater than or equal to a laser oscillation threshold to the induced emission unit and sets one wavelength component of light produced by spontaneous emission as a trigger (excitation light) for induced emission. Therefore, even if the excitation light is supplied by an electrical signal at a high speed up to 100 GHz for inputting transmission information, the induced emission for generating the light signal is followed up as described above. This action is very useful from the viewpoint of the first invention for implementing the speeding up for light transmission and an increase in the capacity for the light transmission by a single wavelength. The merit or response of the follow-up or followability of the induced emission for the light signal to the electrical signal (transmission information) is useful for the purpose of assuredly providing mutual synchronization between light signal generating means provided every wavelengths in terms of the second invention for implementing the speeding up of light transmission and an increase in the capacity for the light transmission. Further, the non-execution of the induced emission by carrier injection is useful for the third invention in terms of the fact that it restrains an increase in the temperature of the light signal generating means and restricts a variation in wavelength to the minimum.
Another optical communication apparatus according to the present invention comprises means for generating light signals according to induced emission, and wavelength selecting means (band pass filter) optically coupled to the light signal generating means. The wavelength selecting means is optically coupled to a light signal transmission line. The light signal generating means has a light emitting portion for generating light according to the injection of a current therein, an induced emission unit for inductively emitting each light signal, and an excitation light generating unit for producing the induced emission. The excitation light generating unit supplies light (excitation light) of a specific wavelength component, which is included in the light produced in the light emitting portion according to the application of an electric field corresponding to transmission information, to the induced emission region. This construction is particularly suitable for an optical communication apparatus using the above-described wavelength modulation scheme (this ground is referred to the description related to FIG. 6). However, if attenuation of some extent occurs in light transmitted through the wavelength selecting means, then the present construction is useful even for the induced emission modulation scheme. Incidentally, the description of the mechanism related to the generation of the light signal will be omitted because it is identical to the aforementioned optical communication apparatus.
In the two types of optical communication apparatuses described above, other optical devices may be provided between the two means and light signal transmission lines constituting them, respectively. When a WDM type optical communication apparatus is constructed, for example, a star coupler or a directional coupler for collectively launching a plurality of light signals different in wavelength into a single light signal transmission line may be provided between the light intensity attenuating means or the wavelength selecting means and the light signal transmission line.
In either of the above-described optical communication apparatuses, the intensity or direction maintains the relation between an electric field applied to the light emitting portion to inject a current into the light emitting portion and an electric field applied to the excitation light generating unit to supply excitation light to the induced emission unit owing to the structure of the light signal generating means. Even in the case of either of the induced emission modulation scheme and wavelength modulation scheme, controlling the voltage amplitude (corresponding to the difference between respective voltage values corresponding to on and off) of an electric field signal applied to the excitation light generating unit so as to be lower than a potential difference applied to the light emitting portion is generally recommended with a view toward causing the excitation light generating unit to have a reflecting function suitable for supplying a desired wavelength component of light produced from the light emitting portion to the induced emission unit. One reason for its recommendation is that even when the transmission rate is increased to 100 Gb/s, the follow-up of the excitation light generating unit to the transmission information can be ensured. This is an advantage based on the operation principle of the present invention capable of overcoming the drawback of the semiconductor optical modulator described in the background art. However, there may be a case where in the induced emission modulation scheme or system, the transmission-rate follow-up of the excitation light generating device can be ensured owing to the configuration of the light signal generating unit even if the electric field applied to the excitation light generating unit is set to greater than the electric field applied to the light emitting portion.
Both the aforementioned optical communication apparatuses can be used for short-distance optical transmission, e.g., information communication between each base station lying within the same city and users and information communications between base stations distant in a range of about several tens of km as they are in configuration. However, when the optical communication apparatus is used in light signal transmission between base stations distant over 100 km or more, for example, light signal amplifying means may be provided between the light intensity attenuating means or the wavelength selecting means and the light signal transmission line to prevent the influence of attenuation of the intensity of the light signal over the light signal transmission line.
When the present invention is viewed from the entire optical communication system constructed inclusive of a signal transmitting unit for inductively emitting a light signal according to transmission information, a light signal transmission line for transmitting the light signal, and a signal receiving unit for receiving a light signal transmitted through the light signal transmission line and thereby reproducing the transmission information, a basic feature thereof resides in a structure wherein the induced emission for the light signal is carried out by excitation light produced according to the transmission information, and the light signal is transmitted at a transmission rate of 1 Gb/s or more between the signal transmitting unit and the signal receiving unit. It is desirable that when the excitation light is intermittently produced by an electric signal corresponding to transmission information, the electric signal is supplied as a voltage pulse. It is desirable to reduce the time width (so-called pulse width) of the voltage pulse to 1xc3x9710xe2x88x929s (seconds) or less. The present structure is an embodiment according to the present invention, which is suitable for the implementation of the speeding up of information transmission by the optical communication system and an increase in the capacity for the information transmission with reduced power consumption.
If the advantage of the optical communication system according to the present invention is revealed from the viewpoint of the industrial applicability, then the present invention resides in that the transmission rate of the light signal per wavelength (i.e., one light signal) could be increased to 10 Gb/s or more by performing the induced emission for the light signal with a light pulse (described specifically, pulse light of having a pulse width of 1xc3x9710xe2x88x929s (seconds) or less) short in time width. Here, the term xe2x80x9clight signal of one wavelengthxe2x80x9d defines an information transfer medium produced by modulating the intensity of light lying in a predetermined wavelength band if strictly spoken. Namely, one wavelength indicates a typical value of a wavelength component of light related to signal transmission and is given for convenience. It is to be noted that a wavelength component, which contributes to one (one type of) light signal is actually distributed in a given wavelength range and the limitation of the wavelength component to a specific wavelength alone is seldom. In the optical communication system according to the present invention, information communications at a transmission rate of 20 Gb/s or more at which a plurality of light signals different in wavelength have heretofore been inevitably used, particularly, transmission rates ranging from 40 Gb/s to 100 Gb/s required of the next-generation optical communication system can be carried out by one type of wavelength signal. Therefore, a high impact is obtained in terms of simplification of the system construction (reduction in the load on maintenance) and an improvement in the management of reliability. When the transmission rate at one wavelength is increased, the followability of a light receiving element or photo-diode on the signal receiving unit side must be considered. Light receiving means having a high-speed response of 10 Gb/s or more has been introduced in Japanese Patent Application Laid-Open No. Hei 5-102515, and means for detecting the waveform of a light signal of a high velocity up to 100 Gb/s has been introduced in Japanese Patent Application Laid-Open No. Hei 8-152361, respectively. It is however recommended that when an avalanche type photo-diode (APD) and a PN type or PIN type photo-diode (PD) whose responsivity are respectively the order of a few Gb/s (in which the maximum value of a transmission rate of responsible light signal falls within a range from above 1 Gb/s to less than 10 Gb/s), are used, a plurality of photo-diodes or light receiving elements are placed in parallel and light signals incident thereto respectively are suitably cut off with time differences defined therebetween. Further, a reference signal for setting the time differences may be transmitted from the signal transmission unit with a wavelength different from that of the light signal.
Even if the present invention is adopted in a system wherein information communications are performed through the use of a plurality of light signals different in wavelength as in the WDM scheme or system, mutual transmission timings can be provided with satisfactory accuracy at a transmission rate of 10 Gb/s or more per wavelength, thereby making it possible to implement a high-reliability optical communication system.
The present invention provides not only the above-described optical communication apparatuses and systems but also it proposes control circuits and light signal transmission modules suitable for them. However, their summaries will be described in detail in best modes for carrying out the invention.